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Energy & Fuels 2008, 22, 4006–4011
Copolymers as Flow Improvers for Mexican Crude Oils Laura V. Castro and Flavio Vazquez* Programa de Ingenierı´a Molecular, Instituto Mexicano del Petro´leo, Eje Central Norte “La´zaro Ca´rdenas” 152, San Bartolo Atepehuacan, C. P. 07730, Mexico, D.F. ReceiVed June 16, 2008. ReVised Manuscript ReceiVed August 18, 2008
A series of copolymers derived from styrene (S), n-butyl acrylate (BuA), and vinyl acetate (VA) monomers were prepared by emulsion polymerization to improve some physical properties of Mexican crude oils (MCOs). Once obtained, the copolymers were characterized by Fourier transform infrared (FTIR) spectroscopy, sizeexclusion chromatography (SEC), and thermogravimetric analysis (TGA). Later, they were dissolved in toluene and were added to both light and heavy MCOs; the pour point temperature and apparent viscosity of these mixtures were carefully evaluated. Results indicate that styrene and vinyl acetate (SVA) copolymers show a good pour point depressant performance for both types of crude oils. The apparent viscosity dependence on temperature for light and heavy crude oils, mixed with copolymers, was also established. It was observed that, in general, the addition of copolymers decreases the apparent viscosity of both light and heavy crude oils above 35 °C.
Introduction Deep water crude oil production poses serious operational challenges, among them the obstruction of production lines. Such lines are permanently immersed in cold seawater. Under these conditions, the temperature drop provokes crystallization of some heavy oil fractions inside the production pipes. The separation of such fractions, mainly constituted by paraffins and waxes, produces solid deposits. These are responsible for the pipes cross-section reduction.1,2 Other constituents in the crude oil (i.e., asphaltenes, resins, lighter distillates methane hydrates, and polar aromatics) should also be considered as important byproducts affecting the flow behavior inside the lines.3 For example, crude oils having high asphaltene contents are also highly viscous and may cause production obstructions when they precipitate near the well bore, because of changes in both pressure and temperature.4 Moreover, asphaltenes play an important role in the crystallization of waxes.5 Flow improving (FI) additives, alternatively known as pour point depressants (PPDs)/wax crystal modifiers, can reduce the growth and size of wax crystals. Thus, the ability of these crystals to flocculate and interlock is greatly diminished. The combination of these effects lowers the pour point, viscosity, and yield stress appreciably, facilitating the transportation of waxy crude oils.3 Many investigations relating to crystallization and additive action mechanisms have been performed by petroleum companies and academic laboratories. However, because of the * To whom correspondence should be addressed. Telephone: +52-5591-75-8402. Fax: +52-55-91-75-7969. E-mail:
[email protected]. (1) Andre´, M. L. C.; Elizabete, L. F.; Gaspar, G. J. Pet. Sci. Eng. 2001, 32, 159–165. (2) Deshmukh, S.; Bharambe, D. P. Fuel Process. Technol. 2008, 89, 227–233. (3) Chanda, D.; Sarmah, A.; Borthakur, A.; Rao, K. V.; Subrahmanyam, B.; Das, H. C. Fuel 1998, 77,11, 1163–1167. ¨ stlund, J.-A.; Chawla, H.; Wattana, P.; Nyden, (4) Venkatesan, R.; O M.; Fogler, H. S. Energy Fuels 2003, 17, 1630–1640. (5) Borthakur, A.; Chanda, D.; Dutta Choudhury, S. R.; Rao, K. V.; Subrahmanyam, B. Energy Fuels 1996, 10, 844–848.
structural complexity of petroleum products, most of these studies have dealt with model mixtures containing well-defined paraffinic compounds and pure solvents. Nevertheless, the interpretation of the results is rather difficult. The wax crystal modifiers are polymeric compounds constituted by both a hydrocarbon chain, which provides the interaction between the additive and paraffin, and a polar segment, responsible for the modification of the wax crystals morphology, necessary to inhibit the aggregation stage.1,2 The crystallization of paraffins in waxy crude oils and petroleum products is governed by three successive phenomena: nucleation, growth, and agglomeration. Some additives promote nucleation, crystallizing before waxes and increasing the number of nuclei. Different wax crystal modifiers can have a specific effect on the crystallization steps.22,25 These inhibitors are not universal ones because their action is effective in a certain molar mass interval, thus requiring experimental tests to assess the best inhibitor product for each particular system.6 The following are the most extensively used flow improvers for both residual fuel and waxy crude oils: highly branched polyR-olefins, ethylene-vinyl acetate copolymers, alkyl esters of unsaturated carboxylic acid-R-olefin copolymers, ethylene-vinyl fatty acid ester copolymers, vinyl acetate-R-olefin copolymers, styrene-maleic anhydride copolymers, long-chain fatty acid amides and poly n-alkyl acrylates,1 and methacrylate copolymers.6-15 They are efficient as crude oil flow modifiers and wax deposition inhibitors. With the declination of conventional production of crude oils (light crude oils) and the necessity to restore the reserves, petroleum companies are actually more interested in heavy crude oils. Particularly, Mexico produces different types of crude oils that are mixed to average their properties to process them without difficulty. For commercial purposes and to ensure a major economic value of Mexican hydrocarbons, the crude oils are sold, principally; on the international merchandise, there are (6) da Silva, C. X.; Alvares, D. R. S.; Lucas, E. F. Energy Fuels 2004, 18 (3), 599–604.
10.1021/ef800448a CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
Copolymers as Flow ImproVers for MCOs
general mixtures of different crude oils. It is important to mention that, concerning the total reserves of Mexican crude oils (MCOs), 55.5% corresponds to heavy crude oil, 35.5% corresponds to light crude oil, and 9.0% are reserves of ultralight crude oil.24 In the present work, a series of copolymers based on different combinations of vinyl acetate (VA), styrene (S), and n-butyl acrylate (BuA), having a wide range of molar mass, were prepared and characterized by spectroscopic, thermal, and chromatographic techniques. These monomers were chosen because the VA functional groups seem to possess high affinity to the paraffinic components in the crude oil, while BuA confers some elastomeric features (low values of glass transition temperatures) to the copolymers, facilitating the diffusion of the additive into the paraffin. Also, the styrene hydrophobic nature improves the additive dispersion in crude oil. A semicontinuous process was specifically developed to synthesize polymers with a potential use as flow improvers in crude oil; it allows for the preparation of homogeneous copolymers and hinders the formation of homopolymers. In addition, samples from two MCOs were characterized by physicochemical methods. Each MCO sample was later mixed with the copolymers to evaluate their efficiency as flow improvers/PPDs. Experimental Section Materials. The following substances were used in this research project. Styrene (Aldrich), n-butyl acrylate (Aldrich), vinyl acetate (Aldrich), initiator (ammonium persulfate, APS, Fermont), buffer (sodium bicarbonate, J. T. Baker), emulsifiers (Abex 26-S, Rhodia and Disponil AES 13 IS, Cognis), and the chain-transfer agent (CTA, 1-dodecanethiol, Aldrich); these were employed with no further purification. The other chemicals used were technical-grade. Ethylene vinyl acetate copolymer (Aldrich), with a composition E-VA ) 75:25, was used as received. Polymerizations. Styrene-vinyl acetate copolymers cannot be synthesized in a batch reactor because of their low tendency of vinyl acetate to react with styrene (r1r2 ) 2.4016), which hinders the formation of random copolymers. A simple addition of both monomers into the reactor will finally produce a mixture of polyvinyl acetate and polystyrene chains. A synthesis procedure in a semicontinuous reactor, introducing both monomers in the main reactor under strict starved-feed conditions (with the monomer feed rate lower than the polymerization rate), is absolutely necessary. A slow monomer addition into the main reactor favors the copolymerization of S and VA and hinders composition derivations during the reaction. In contrast, the other combinations of monomers (BuA-VA and S-BuA) included in this study exhibited a clear tendency to copolymerize. However, to ensure a homogeneous copolymerization, all reactions were performed in a semicontinuous device. The main reactor consisted of a 1000 mL five-necked flask equipped with a heating jacket system, a reflux condenser, and a (7) EI-Gamal, I. M.; Atta, A. M.; Al-Sabbagh, A. M. Fuel 1997, 76 (14/15), 1471–1478. (8) Pedersen, K. S.; Rønningsen, H. P. Energy Fuels 2003, 17, 321– 328. (9) Song, Y.; Ren, T.; Fu, X.; Xu, X. Fuel Process. Technol. 2005, 86, 641–650. (10) Andre´, M. L. C.; Lucas, F. Pet. Sci. Technol. 2001, 19 (1 and 2), 197–204. (11) Ahlers, W.; et al. U.S. Patent Application 20070094920, 2007. (12) Bloch, R. A.; Martella, D. J. U.S. Patent 6,475,963, 2002. (13) Balzer, J.; Feustel, M.; Krull, M.; Reimann, W. U.S. Patent 5,439,981, 1995. (14) Wahle, B.; Herold, C.-P.; Zoellner, W.; Schieferstein, L.; Oberkobusch, D. U.S. Patent 5,006,621, 1991. (15) Tack, R. D.; Andrews, R. F.; Ayres, S. J. U.S. Patent 4,874,394, 1989. (16) Brandrup, J.; Immergut, E. H.; Grulke, E. A. Polymer Handbook, 4th ed.; John Wiley and Sons: New York, 1999; p II/274.
Energy & Fuels, Vol. 22, No. 6, 2008 4007 Table 1. Typical Formulation for the Preparation of Polymer Latexes substance
main reactor (g)
feeding tank (g)
monomer mixture surfactant solution (Abex, 7 wt %) surfactant solution (Disponil, 7 wt %) initiator solution (APS, 4 wt %) buffer solution NAHCO3 (4 wt %) chain-transfer agent (CTA) deionized water
0 5 5 5 25 0 100
100 67 67 20 0 8 98
mechanical stirrer. A Masterflex Model 77201-60 peristaltic pump was used to feed the reactor from a stirred glass tank containing the pre-emulsified monomers (q˙ ) 7.2 × 10-3 g mL-1 min-1). The polymerizations were conducted at T ) 70 ( 0.05 °C under nitrogen atmosphere. All reagents, corresponding to the initial main reactor load (Table 1), were charged at the beginning of the polymerization, except the initiator solution. The reactor was heated and maintained at the reaction temperature for 10 min, and then the initiator solution was added. A total of 10 min later, the pre-emulsion was added from the feeding tank with the aid of the peristaltic pump. To operate under starved-feed conditions and hinder compositional derivations, the addition time was limited to 100 min. After the addition time, the latex was maintained at 70 °C for 2 h to reduce the residual monomer, and then it was finally cooled at room temperature. The formulation shown in Table 1 has been used to obtain latexes with a 20 wt % solid content. The copolymers were isolated by water evaporation and dried at room temperature. They showed either yellow viscous liquid or pale yellow wax-like appearances. The polymers molar mass and polydispersity were controlled by introducing small amounts of 1-dodecanethiol (CTA) as a chaintransfer agent. Copolymer Characterization. Size-Exclusion Chromatography (SEC). The number-average molar mass Mn and weight-average molar mass Mw were determined by SEC, which is a experimental method based on the size distribution and number of molecules in a polymer sample. The number and weight average molar mass were calculated according to the following equations:
∑N M ∑N ∑N M ) ∑N M
Mn )
x
x
(1)
x
Mw
2
x
x
x
x
(2)
where the summations are over all of the different sizes of polymer molecules from x ) 1 to ∞ and Nx is the number of moles, whose molar mass is Mx.17 The polydispersity index, I ) Mw/Mn, is a measure of the broadness of the molar mass distribution. The molar mass and polydispersity index (I) of copolymers were determined by using a SEC Agilent 1100 chromatograph with a 5 µm column of PLgel. Tetrahydrofuran, at flow rate of 1 mL min-1 at 30 °C, was used as the mobile phase. Calibration was made with a polystyrene standards kit. Fourier Transform Infrared (FTIR) Spectroscopy. For the FTIR studies, a Thermo Nicolet AVATAR 330 FTIR device was used to obtain the spectra from a dilute solution of copolymer and dichloromethane through a KCl cell. It was equipped with a DTGS KBr detector with a KBr beamsplitter. The solvent was evaporated; therefore, the copolymer spectrum was integrated by the spectrometer. A total of 32 scans were taken for each sample recorded from 4000 to 400 cm-1 in the transmission mode. The spectrum resolution used was 4 cm-1. (17) Odian, G. Principles of Polymerization; John Wiley and Sons: New York, 2004; pp 21-22.
4008 Energy & Fuels, Vol. 22, No. 6, 2008
Castro and Vazquez
Table 2. Average Molar Mass and Polydispersity Index of Copolymers molar mass by SEC monomer A
monomer B
copolymer
monomer ratio (g/g)
Mn (g/mol)
Mw (g/mol)
I
styrene n-butyl acrylate styrene styrene ethylene
n-butyl acrylate vinyl acetate vinyl acetate vinyl acetate vinyl acetate
S-BuA BuA-VA LMWS-VA HMWS-VA E-VA
S/BuA ) 70:30 BuA/VA ) 70:30 S/VA ) 70:30 S/VA ) 70:30 E/VA ) 75:25
2938 2213 1691 9279 38678
4025 2678 2486 19300 116034
1.37 1.21 1.47 2.08 3.00
Differential Scanning Calorimetry (DSC). The glass transition temperatures (Tg) were determined using a Shimadzu DSC-60A differential scanning calorimeter with a TA-60WS interface. Tg is often used as a measure of polymers hardness and, for a copolymer, is mainly dependent upon the monomers used to prepare it. In several cases, the Tg of a copolymer, terpolymer, etc., can be approximated by the Fox equation18
wA wB 1 ) + + ... Tg TgA TgB
(3)
where wA is the weight fraction of monomer A in the copolymer, wB is the weight fraction of monomer B, and so on. A calibration procedure was performed using an Indium standard. The apparatus was continually flushed with nitrogen. Each sample (between 10 and 20 mg) was cooled to either -25 or -70 °C for 3 min, and then it was heated at a rate of 8 °C/min until reaching 120 °C. ThermograVimetric Analysis (TGA). Thermal stability was studied by TGA measurements using a TA-instruments TGA 2950 device. Each sample (between 10 and 15 mg) was heated from 20 to 500 °C at a rate of q˙ ) 5 °C/min. The apparatus was continually flushed with a nitrogen flow of 10 mL/min. A calibration procedure was performed using calcium oxalate monohydrate (CaC2O4 · H2O) in a temperature range between 20 and 800 °C. Crude Oil Characterization. Physical Testing. Specific gravity and kinematic viscosity were determined according to the ASTM-D 287 and ASTM-D 445 methods, respectively. The average molar mass were determined by freezing point depression of decanesaturated benzene solutions using a petroleum cryoscope (Cryette No. 1 Model 5009). Pour points were evaluated according to the ASTM-D 97 method without reheating to 46 °C. Sulfur was measured as directed by the ASTM-D 4294 standard. Wax and asphaltene contents were determined according to the UOP-46 norm and the ASTM-D 2007 method, respectively. AdditiVe Preparation. The additives consisted of 100 mL of toluene containing 0.3 g of each copolymer; each solution was stirred for 10 min at a doping temperature for homogenization.
Figure 1. FTIR spectra of copolymers.
AdditiVe Treatment. To obtain consistent results with accurate rheological measurements, the memory of the evaluated crude oil samples has to be removed by heating at 80 °C while stirring.19 Tests started by heating the preconditioned untreated crude oil samples to 60 °C in an ultrasonic bath and then loading them into a hermetic bottle with an appropriate amount of flow improver. Finally, the temperature of the sample in the ultrasonic bath was maintained at 60 ( 1 °C for 30 min. Viscosity Measurements. Kinematic viscosity was measured using Ubbelohde glass capillary viscometer tubes with appropriate capillar constants.19 Measurements were carried out within 25 and 50 °C, at 5 °C intervals. An average of four parallel measurements at each temperature was reported. Repeatability wise, the relative error generally was within 0.5-1%. The kinematic viscosity (ν in units of mm2 s-1) is related to the dynamic viscosity (µ in units of mPa s or cP) as follows:
υ ) µ/F
(4)
where F is the density in g/cm3.
Results and Discussion Five different FI additives were prepared and tested in MCO. Four of them were copolymers, synthesized by semicontinuous emulsion polymerization for this study. Here, a conversion near 100% was determined by gravimetric measurements. The molar mass and polydispersity data obtained are reported in Table 2. Chemical–Structural Characterization of Copolymers. The chemical structures of the prepared copolymers were studied through FTIR spectral analysis. The spectrum of each copolymer is shown in Figure 1. FTIR spectra exhibit the characteristic bands of synthesized copolymers. The S-BuA, HMWS-VA, and LMWS-VA copolymers spectra clearly show the peaks corresponding to both the aromatic ring and CH2 groups stretching vibrations of the polymerized styrene at 3027 and 2870 cm-1, respectively. It
Copolymers as Flow ImproVers for MCOs
Energy & Fuels, Vol. 22, No. 6, 2008 4009
Table 3. Thermal Behavior of Copolymers from DSC and TGA Measurements
copolymer sample
T g, calculate21
Tg, glass transition (°C)
S-BuA BuA-VA LMWS-VA HMWS-VA E-VA
36.34 -32.58 74.87 74.87 -59.32
17.22 -32.36 81.97 74.94 -52.50
TDi, initial temperature of degradation (°C)
TDf, final temperature of degradation (°C)
276 265 230 230 320
432 460 443 435 480
also shows the aromatic compound CdC stretching vibration peaks at 1604 and 1495 cm-1 and the C-H bending vibrations at 727 and 895 cm-1 in the corresponding spectra. The copolymerization of vinyl acetate was evidenced by the C-H and CH3 absorption peaks caused by deformation and twisting at 2920, 1370, and 1450 cm-1, respectively. Other VA expected signals clearly detected were the CdO strong absorption peak at 1730 cm-1 and the asymmetric stretching of C-O-C at 1240 cm-1. The flexion of C-H at 2980 cm-1, the characteristic CdO strong absorption peak at 1730 cm-1, and the asymmetric stretching of C-O-C at 1170 cm-1 of the S-BuA and BuA-VA spectra all correspond to the n-butyl acrylate units in the chains. It must be remarked that vibrational frequencies of the -CdC- bonds in vinyl compounds, whose peaks are at 1660-1640 cm-1, were not detected in any spectrum.20 Thermal Analysis of Copolymers. Glass transition temperatures were determined from DSC measurements as midpoints in the glass transition region and are reported in Table 3. The copolymers obtained with BuA and E-VA exhibited low Tg value and should be considered as elastomers. It is important to outline that all of the thermograms exhibit a single glass transition zone, confirming that the synthesized copolymers are random in nature and have not compositional derivations.21 Random copolymers exist as a single phase and must show a single Tg, which in turn depends upon its monomer content. Thus, the temperature at which the glass transition occur provides information about the copolymer structure and composition. The differences between the experimental and calculated copolymer glass transitions are quiet similar, except for the copolymer sample S-BuA. TGA measurements reveal that the copolymers synthesized by emulsion polymerization are thermally stable until 160 °C (Figure 2) and that the degradation of the copolymer backbone was below 400 °C. The commercial copolymer (E-VA) thermogravimetric curve revealed that this copolymer is stable until 300 °C. Its ulterior degradation is governed by two mass losses: the first at 357 °C, and the second, a complete material degradation of the copolymer backbone, at 480 °C. The TGA study leads to affirm that there is no degradation of the products within the temperature range of the crude transportation. Table 3 shows the percentage of mass loss and the initial and final degradation temperatures of the copolymer. MCO Characterization. The MCOs were supplied by the Mexican Petroleum Company (PEMEX), and their physico(18) Bicerano, J. Prediction of Polymer Properties; Marcel Dekker, Inc.: New York, 1993. (19) Ronningsen, H. P.; Bjoerndal, B.; Hansen, A. B.; Pedersen, W. B. Energy Fuels 1991, 5, 895–908. (20) Gauglitz, G.; Vo-Dinh, T. Handbook of Spectroscopy; Wiley-VCH: Germany, 2003. (21) Hatakeyama, T.; Quinn, F. X. Thermal Analysis Fundamentals and Applications to Polymer Science; John Wiley and Sons: New York, 1994. (22) Geiza, E. O.; Mansur, C.; Lucas, E. F. J. Dispersion Sci. Technol. 2007, 28, 349–356.
Figure 2. Thermograms of copolymers tested as flow improvers of MCO. Table 4. Physical Characteristics of Crude Oils Tested crude oils parameter
light
heavy
specific gravity at 60/60 °F kinematic viscosity (mm2 s-1) at 25 °C molar mass (g/mol) pour point (°C) sulfur (wt %) wax content (wt %) asphaltene content from n-C5 (wt %)
0.8784 15.46 240.8 -21 1.941 3.02 8.44
0.9262 198.56 314.8 -24 3.397 3.83 15.7
Figure 3. Effect of the additive concentration on the pour point of light MCO for different copolymers.
chemical characteristic data are provided in Table 4. Both crude oils contain appreciable amounts of asphaltenes, which strongly influences their flow behavior. Effect of Additives on the Pour Point of MCO. The five copolymers used in this project were assessed as improvers of the MCO cold-flow properties under pour point and viscosity testing. The E-VA copolymer was included to compare results with a commercial PPD. Once prepared, the additives were tested at different concentrations ranging from 200 to 4000 ppm. The results are reported in Figures 3 and 4. The lowest pour point depression registered for the light crude oil sample was achieved by the LMWS-VA and E-VA copolymers. Because of the differences in the structure and physicochemical characteristics, the polymer efficiencies as PPDs varied. However,
4010 Energy & Fuels, Vol. 22, No. 6, 2008
Figure 4. Effect of additive concentration on pour point of heavy MCO for different copolymers.
the contribution of the acetate group, hindering the paraffin crystallization in the crude oil, must be detected as a decrement of the pour point. In addition, the average molecular-weight effect was detected from the behavior of LMWS-VA and HMWS-VA. Indeed, these two vinyl copolymers showed better efficiency as PPDs than the S-BuA copolymer. This may be explained in terms of a faster sliding of the short chains on both S-VA through waxes and paraffins, being more apparent in the case of the LMWS-VA additive. The copolymers S-BuA and BuA-VA had little or null effect on the pour point of the crude oils despite the low Tg of these copolymers. This may be explained by the lack of interaction between the copolymer unit n-butyl acrylate and the components of the crude oil sample, especially with the waxes, whereas the LMWS-VA and HMWS-VA copolymers caused an improved effect on the crude oils. The pour point was reduced considerably at dosages of S-VA and E-VA between 500 and 2000 ppm. A large pour point decrement of the light crude oil was obtained by using E-VA. This could be explained in terms of the compatibility between this copolymer and the considerable number of waxes and paraffins in this kind of petroleum. However, the LMWS-VA showed a better performance when tested in a heavier crude oil, one with a large content of aromatic compounds and more content of asphaltenes. This result indicates that asphaltene-rich crude oils requires lower molar mass copolymer as a PPD. Some authors3 indicate that asphaltenes act as natural flow improvers for crude oils. Asphaltenes concentrate on wax crystals and avoid the formation of large crystals. The crystal grow rate of the lower molar mass LMWS-VA is much slower than that of the higher molar mass HMWS-VA. According to this information, it may be concluded that asphaltene aggregation and the presence of aliphatic groups at the surface of these aggregates are essential features for the inhibition of wax crystal growing and aggregation.22 With all of the copolymers tested, it was possible to show the effect of copolymer composition (principally the presence of vinyl acetate and n-butyl acrylate) and the effect of molar mass in the copolymers of S-VA. Effect of Additives on the MCO Sample Rheology. The five copolymers were tested by viscosimetry (BuA-VA, S-BuA, HMWS-AV, E-VA, and LMWS-VA) to assess their influence on the rheological behavior of MCOs, this at a temperature higher than the pour point as described in the Experimental Section. The apparent viscosity versus temperature
Castro and Vazquez
Figure 5. Temperature effect on the apparent viscosity of light MCO containing 1000 ppm of FI.
Figure 6. Temperature effect on the apparent viscosity of heavy MCO containing 1000 ppm of FI.
data of the untreated and additive-treated MCOs are plotted in Figures 5 and 6. The copolymers developed for this test presented a satisfactory, although limited, efficiency as viscosity reducers. In Figure 5, the data suggest that the S-BuA and BuA-VA copolymers were more effective as flow improvers for the light crude oil sample, only at temperatures above 35 °C. In contrast, it was observed that lower molar mass copolymers are less effective as flow improvers for light MCO. Conversely, the corresponding data for heavier crude oil suggest that the four macromolecules, synthesized by emulsion polymerization, were more efficient flow improvers as compared to the commercial E-VA for temperatures above 35 °C. The LMWS-VA and HMWS-VA copolymers were effective as FI in all temperatures tested, despite the different molar mass of both copolymers. An important parameter to reduce the crude oil viscosity and enhance the flowability is the temperature manipulation. The temperature level strongly influences the viscosity of the high molar mass components in crude oils. Raising the temperature of the crude oil to high levels dis-establishes the ordered structures of the crude oil heavy components, which will severely reduce the viscosity.
Copolymers as Flow ImproVers for MCOs
The diverse influences of the additives tried in this project on both heavy and light crude oils is easily explained in terms of their distinctive compositional parameters (asphaltenes fraction, saturates fraction, sulfur content, etc.). It has been reported that additives usually are not equally effective when tested in diverse crude oils and sometimes not effective at all.23 The flow improver addition to enhance crude oil flowability is not recommended for light crude oils because of the limited reduction of viscosity observed in those fluids. In contrast, the addition of 1000 ppm of a S-VA flow improver into heavy crude oil reduced the viscosity between 9 and 26% along the whole temperature interval from 30 to 50 °C (Figure 6). It was also observed that the influence of the copolymer (S-VA) molar mass on the viscosity reduction of heavy crude oil is very limited. The reduction of viscosity with the addition of copolymers with n-butyl acrylate was increased after the temperature of 35 °C, and its behavior was similar at the copolymer S-VAs. For MCOs, E-VA copolymer is a good PPD but not a suitable viscosity reducer. When there are considerable quantities of asphaltenes on crude oils, the rheological behavior of the E-VA structure is reduced. Conclusions Four copolymers were synthesized by emulsion polymerization techniques and then assessed as flow improvers for two (23) Ghannam, M. T.; Esmail, M. N. Pet. Sci. Technol. 2006, 24, 985– 999.
Energy & Fuels, Vol. 22, No. 6, 2008 4011
MCOs. The pour point evaluation of the as-prepared copolymers, added into light and heavy MCOs, revealed that the E-VA copolymer showed the best performance as a PPD. In contrast, the copolymers containing n-butyl acrylate had little or not effect as pour point depressors, when compared to VA copolymers. The effect of molar mass on the copolymers is important for a PPD. A second testing series based on determinations of apparent viscosity as a function of temperature showed a more complex behavior of the copolymers dispersed in the MCOs. Despite its good efficacy as a PPD, the E-VA copolymer contributed to improve the flow of the light MCO only in the high-temperature range. In contrast, it was clearly observed that low molar mass copolymers (LMWS-VA) may be used as PPDs for light and heavy MCOs containing (>8 wt %) asphaltenes and (
